Exposure to the bright light in morning could help weight control, researchers say, who found the intensity, duration and -- most importantly -- the timing of light exposure during the course of a day is linked to person's weight.

A study at Northwestern University found people whose daytime exposure to bright light occurred mostly in the morning hours possessed a noticeably lower BMI (body mass index) than those whose predominant light exposure happened later.

Morning light's exposure effect on weight was found to be independent of a person's age, level of physical activity, sleep timing, caloric intake or the season, the university researchers say.

"Light is the most potent agent to synchronize your internal body clock that regulates circadian rhythms, which in turn also regulate energy balance," lead study author Phyllis C. Zee says. "The message is that you should get more bright light between 8 a.m. and noon."

Just a half hour of morning bright light is sufficient to have an effect on BMI, the study found.

The lifestyle of most Americans has them predominantly indoors, many people get very little natural sunlight during the morning, Zee says and many people work in environments with poor lighting.

Such indoor lighting is usually measured between 200 lux and 300 lux, whereas 500 lux is considered a minimum intensity for possessing a lower body mass index, researchers say.

Compared to indoor lighting, outdoor lighting, even on cloudy days, has a brightness level exceeding 1,000 lux.

"Light is a modifiable factor with the potential to be used in weight management programs," study co-author and Northwest neurologist Kathryn Reid says. "Just like people are trying to get more sleep to help them lose weight, perhaps manipulating light is another way to lose weight."

During the study, 54 participants evenly divided between males and females wore wrist monitors to track light exposure and sleep amounts over a seven-day period, while their caloric intake was traced through food logs.

"If a person doesn't get sufficient light at the appropriate time of day, it could de-synchronize your internal body clock, which is known to alter metabolism and can lead to weight gain," Zee says.

However, the exact mechanism wherein light affects a person's BMI deserves further research.

"We focus on how too much light at night is bad; it's also bad not to get enough light at the appropriate time during the day."